We propose to define the structural and functional properties of those active ion transport processes of the small intestine that produce osmotic work (secretion and absorption of fluid) and to identify the factors (cyclic nucleotides, hormones, extracellular HCO3 and pH and others) regulating these processes. Through measurements of (a) radioisotope fluxes into and across the epithelium, (b) membrane phosphorylation and dephosphorylation and (c) certain membrane enzyme activities (ATPases, adenylate and guanylate cyclases), we hope to gain insight into the specific roles of villus cells as opposed to crypt cells and of the basolateral cell border as opposed to the luminal cell border. We will attempt to purify in undenatured or renaturable form relevant membrane phosphoproteins and ATPases and then to reconstitute transport systems by incorporating these proteins into phospholipid vesicles. To gain further insight into these processes and their regulation, studies will be carried out on small intestine from two phylogenetically separated species--a mammal (rabbit) and a teleost (eel or flounder) that can adapt to both a fresh and salt water existence.